The present invention relates to a supported metallocene catalyst system having a modified support useful for the polymerization of olefins. More particularly, but not by way of limitation, the present invention relates to a supported metallocene system having supports modified by more than one drying condition. In another aspect the present invention relates to a method for polymerization of olefins with a broad molecular weight distribution using the inventive supported metallocene catalyst system.
Metallocene catalyst systems are extensively used in a variety of polymerization systems, including the polymerization of olefins. The term xe2x80x9cmetallocenexe2x80x9d as used herein refers to a compound containing at least one cyclopentadienyl-type group bonded to a transition metal. The transition metal is selected from Groups IVB, VB, and VIB, preferably IVB and VIB. Examples include titanium, zirconium, hafnium, chromium and vanadium. Generally, the more preferred catalysts in the polymerization of olefins are metallocenes of Zr, Hf, or Ti.
Generally, in order to obtain the highest activity from metallocene catalysts, it has been necessary to use them with an organoaluminoxane cocatalyst, such as methylaluminoxane. The resulting catalyst system is generally referred to as a homogenous catalyst system since at least part of the metallocene or the organoaluminoxane is in solution in the polymerization media. These homogenous catalysts systems have the disadvantage that when they are used under slurry polymerization conditions, they produce polymer which sticks to reactor walls during the polymerization process (generally referred to as xe2x80x9cfoulingxe2x80x9d) and/or polymer having small particle size and low bulk density which limits the commercial utility.
Various methods have been proposed in an effort to overcome the disadvantages of the homogenous metallocene catalyst systems. One such method involves the prepolymerization of the metallocene aluminoxane catalyst system and/or supporting the catalyst system components on a porous carrier (also known as a xe2x80x9cparticulate solidxe2x80x9d or xe2x80x9csupportxe2x80x9d).
Another important consideration in development of metallocene catalysts is the molecular weight distribution of the solid polymer generated using such catalysts. Broad molecular weight distributions have been linked to improved processability and improved performance of the resulting polymer. Although there have been numerous approaches to broadening the molecular weight distribution of polymers generated from metallocene catalyst systems, there continues to be a need for improved techniques and catalysts.
One aim of the present invention is to provide a new method for preparing a supported metallocene catalyst system capable of producing polyolefins having a broad molecular weight distribution. In accordance with another aspect of the present invention, a method is provided for polymerizing olefins using the new type of supported metallocene catalyst system.
The present invention provides a process for preparing a supported metallocene catalyst system that can be used to catalyze formation of polyolefins having a wide molecular weight range. In accordance with the process of the present invention, a first portion of an inorganic oxide is dried under a first set of conditions, wherein upon treatment with an organoaluminoxide and a metallocene, a first catalyst system is formed usable to produce a polyolefin product having a first molecular weight average distribution. A second portion of an inorganic oxide under a second set of conditions, wherein upon treatment with an organoaluminoxane and a metallocene, a second catalyst system is formed usable to produce polyolefin product having a second molecular weight distribution. An amount of the first portion is mixed with an amount of the second portion to produce a final inorganic oxide support. The final inorganic oxide support is combined with an organoaluminoxane and a metallocene to produce a supported metallocene catalyst system usable to catalyze formation of polyolefins with a broadened molecular weight average distribution.
In an alternative embodiment, the inventive metallocene catalyst system is prepared by drying a first portion of an inorganic oxide under a first set of conditions and combining the first portion with an organoaluminoxane and a metallocene to create a first catalyst system usable to produce a polyolefin product having a first molecular weight average distribution. A second portion of an inorganic oxide is dried under a second set of conditions and combined with an organoaluminoxane and a metallocene to create a second catalyst system usable to produce a polyolefin product having a second molecular weight average distribution. An amount of the first catalyst system can be mixed with an amount of the second catalyst system to form a combined catalyst system usable to catalyze the formation of polyolefins having a broadened molecular weight average distribution.
In another aspect, the present invention provides a process for polymerizing polyolefins using the inventive supported metallocene catalyst system. The process comprises preparing the final catalyst as mentioned above and then contacting an olefin with the final metallocene catalyst system under conditions supporting polymerization.
The present invention provides a particulate solid or xe2x80x9csupport,xe2x80x9d useful in forming a liquid catalyst system for the polymerization of olefins. Any number of supports can be employed as the particulate solid to be modified. Typically the support can be any inorganic oxide, including metal oxides such as silica, alumina, silica-alumina, and mixtures thereof. Other examples of inorganic oxides are magnesia, titania, zirconia, and the like. It is within the scope of the present invention to use a mixture of one or more of the particulate solids.
The present inventors have observed that the extent and type of drying influences the molecular weight distribution of polyolefin product. Combining metallocene catalysts with inorganic oxide dried under varying conditions allows a proportional modification of the molecular weight distribution of the catalyzed polyolefin. Thus, partially dried and more extensively dried inorganic oxide-metallocene catalyst preparations can be mixed in amounts effective to catalyze formation of desired molecular weight distributions of polyolefin, e.g. polyethylene, products. The polyolefin catalyzed by the mixed catalyst preparations will have a broader molecular weight distribution than polyolefin produced by either catalyst system alone. Catalysts prepared under varying drying conditions may be combined before or after TMA treatment in order to achieve a hybrid polyolefin product.
In accordance with a preferred embodiment of the present invention, the solid support is carefully dehydrated to a desired extent prior to use. Preferably for substantially complete dehydration, the solid is dehydrated so as to contain less than 7% weight loss on ignition. Complete or partial dehydration treatment may be carried out in substantial vacuum or while purging with a dry inert gas such as nitrogen or dry air at a temperature of about 20xc2x0 C. to about 1200xc2x0 C., and preferably, from about 300xc2x0 C. to about 800xc2x0 C. for substantially complete dehydration. Pressure considerations are usually not critical. The duration of thermal treatment can be from about 0.1 to about 24 hours, depending on the extent of thermal treatment desired.
In a preferred embodiment, the inorganic oxide support is placed in a ceramic tray which is then placed in a muffle furnace with ambient air. The temperature of the muffle furnace is increased at a rate of 10xc2x0 C. per minute until the desired temperature is reached. The silica is left at the indicated temperatures for the desired time period, depending on the extent of dehydration desired. After removal from the muffle furnace the heated silicon dioxide is placed in a sealed container for further use.
Dehydration is preferably also accomplished by subjecting the solid to a chemical treatment to remove water and reduce the concentration of surface hydroxyl groups. Chemical treatment is generally capable of converting substantially all water and hydroxyl groups in the oxide surface to relatively inert species. Useful chemical agents are for example, trimethylaluminum, ethyl magnesium chloride, chlorosilanes such as SiCl4, disilazane, trimethylchlorosilane, (dimethylamino)trimethylsilane and the like. Trimethylaluminum (xe2x80x9cTMAxe2x80x9d) is a particularly preferred chemical agent.
The chemical dehydration can be accomplished by slurrying the inorganic particulate material such as, for example silica, in an inert low boiling hydrocarbon, such as for example, hexane. During the chemical dehydration treatment, the silica should be maintained in a moisture and oxygen free atmosphere. A low boiling inert hydrocarbon solution of the chemical dehydrating agent is then added to the silica slurry, such as, for example TMA. The solution is added slowly to the slurry. The temperature ranges during chemical dehydration reaction can be from about 20xc2x0 C. to about 120xc2x0 C., however, higher and lower temperatures can be employed. Preferably, the temperature will be from about 50xc2x0 C. to about 100xc2x0 C. The chemical dehydration procedure may be allowed to proceed until all the substantially reactive groups are removed from the particulate support material as indicated by cessation of gas evolution. Normally, the chemical dehydration reaction will be allowed to proceed from about 30 minutes to about 16 hours. Upon completion of the chemical dehydration, the solid particulate material may be filtered under a nitrogen atmosphere and washed one or more times with a dry, oxygen free inert solvent. The wash solvents as well as the diluents employed to form the slurry and the solution of chemical dehydrating agent can be any suitable inert hydrocarbon. Illustrative of such hydrocarbons are pentane, heptane, hexane, toluene, isopentane and the like.
Applicants believe that, like other forms of drying, the extent of chemical drying will affect the molecular weight distribution of the ultimate polyolefin products. Furthermore, it is believed that thermal dehydration can be dispensed with completely in accordance with the present invention. In such a case, the extent of dehydration will be controlled completely by the chemical drying of the support.
The specific particle size of the support or inorganic oxide, surface area, pore volume, and number of hydroxyl groups is not considered critical to its in the practice of this invention. However, such characteristics often determine the amount of support to be employed in preparing the catalyst compositions, as well as affecting the particle morphology of polymers formed. The characteristics of the carrier or support must therefore be taken into consideration in choosing the same for use in the particular invention.
The modified support, prepared as described above, is useful in forming a supported metallocene catalyst system. The supported catalyst system is prepared by contacting the modified support material with a solution of a metallocene catalyst component, which has been treated with an organoaluminoxane as described more filly below, and with ethylene.
A wide range of metallocenes is considered to be applicable to the present process. Particularly preferred metallocenes useful for this invention are those metallocenes discussed in U.S. Pat. No. 5,498,581, assigned to the assignee of the present invention. The metallocenes of the type contemplated as useful for the present invention include those represented by the formula:
xe2x80x83Rx(Z)(Z)MeQk
wherein each Z is bound to Me, is individually selected and is a cyclopentadienyl-type ligand selected from substituted or unsubstituted cyclopentadienyl, indenyl, benzoindenyl, tetrahydroindenyl, benzofluorenyl, octahydrofluorenyl, and fluorenyl ligands, as well as derivatives thereof;
wherein R is a structural bridge linking the Z""s and is selected from the group consisting of hydrocarbyl groups, hydrocarbyloxy groups, silicon containing hydrocarbyl groups, germanium containing hydrocarbyl groups, tin containing hydrocarbyl groups, phosphorus containing hydrocarbyl groups, and nitrogen containing hydrocarbyl groups each having 1 to 20 carbon atoms;
wherein Me is a metal selected from the group consisting of Group IVB, VB, and VIB metals of the Periodic Table;
wherein each Q is individually selected from the group consisting of hydrogen, halogen, hydrocarbyl groups having 1 to 20 carbon atoms, alkoxy groups having 1 to 20 carbon atoms, amino groups which may or may not be substituted with up to two hydrocarbyl groups having 1 to 20 carbon atoms, a phosphorus-containing hydrocarbyl group having 1 to 20 carbon atoms, a silicon-containing hydrocarbyl group having 1 to 20 carbon atoms and an aluminum-containing hydrocarbyl group having 1 to 20 carbon atoms;
wherein x is 1 or 0; and
wherein k is a number sufficient to fill out the remaining valences of Me.
Preferred metallocenes include those metallocenes which contain a substituent which is copolymerizable with an olefin. An even more preferred group would be bridged metallocenes in which the polymerizable substituent is on the bridge. Examples of such polymerizable substituents include alkenyl and alkynel groups. Although bridged metallocenes are preferred to unbridged metallocenes, both are considered applicable to the present invention. Non-limiting examples of bridged metallocenes having fluorenyl-containing components and methods for making same are disclosed in U.S. Pat. No. 5,594,078 which is fully incorporated herein by reference.
The term fluorenyl as used herein refers to 9-fluorenyl unless specifically indicated as otherwise. Accordingly, the term fluorenyl and 9-fluorenyl should be viewed as equivalent unless indicated otherwise.
The organoaluminoxane component used in preparing the inventive solid catalyst system is an oligomeric aluminum compound having repeating units of the formula: 
Some examples are often represented by the general formula:
(Rxe2x80x94Alxe2x80x94O)n
or
R(Rxe2x80x94A1xe2x80x94Oxe2x80x94)nAlR2.
In the general aluminoxane formula, R is a C1-C5 alkyl radical, for example, methyl, ethyl, propyl, butyl or pentyl and xe2x80x9cnxe2x80x9d is an integer from 1 to about 50. Most preferably, R is methyl and xe2x80x9cnxe2x80x9d is at least 4. Aluminoxanes can be prepared by various procedures known in the art. For example, an aluminum alkyl may be treated with water dissolved in an inert organic solvent, or it may be contacted with a hydrated salt, such as hydrated copper sulfate suspended in an inert organic solvent, to yield an aluminoxane. Generally the reaction of an aluminum alkyl with a limited amount of water is postulated to yield a mixture of the linear and cyclic species of the aluminoxane.
Before being contacted with the modified support, the metallocene and aluminoxane are combined in the presence of a suitable liquid to form a liquid catalyst system. The amount of aluminoxane and metallocene used in forming the liquid catalyst system can vary over a wide range. Typically, however, the molar ratio of aluminum in the aluminoxane to transition metal of the metallocene is in the range of about 1:1 to about 20,0000:1, more preferably, a molar ratio of about 50:1 to about 600:1 is used.
It is preferred that the liquid catalyst system be prepared using an organic liquid in which the aluminoxane is at least partially soluble. The currently preferred liquids are hydrocarbons such as hexane or toluene. Typically, an aromatic liquid solvent is employed. Examples include benzene, toluene, ethylbenzene, diethylbenzene, and the like. The amount of liquid to be employed is not particularly critical. Nevertheless, the amount should preferably be such as to dissolve the product of the reaction between the metallocene and the aluminoxane, to provide desirable polymerization viscosity for pre-polymerization, and to permit good mixing. The temperature is preferably kept below that which would cause the metallocene to decompose. Typically the temperature would be in the range of xe2x88x9250xc2x0 C. to 100xc2x0 C. Preferably, the metallocene, the aluminoxane, and the liquid diluent are combined at room temperature, i.e. around 10xc2x0 to 30xc2x0 C. The reaction between the aluminoxane and the metallocene is relatively rapid. The reaction rate can vary depending upon the ligands of the metallocene. It is generally desired that the aluminoxane and metallocene be contacted for about one minute to about one hour. Upon completion of the reaction, a liquid catalyst system will have been formed.
The liquid catalyst system is contacted with the modified support by mixing the liquid catalyst system with the above modified support. Preferably, the liquid catalyst is added to the support with vigorous stirring. The modified support is used in an amount such that the weight ratio of the metallocene to the support is in the range of about 0.00001/1 to 1/1, more preferably 0.005/1 to 0.05/1. The supported catalyst system is precipitated and washed with hexane or another suitable solvent. The catalyst system prepared in this manner to have a modified support yields polypropylene productivity increases of more than a factor of two.
Even further polyolefin productivity increases may be realized within the scope of the present invention by carrying out prepolymerization in the presence of the supported catalyst system. The prepolymerization is conducted in the supported catalyst system, which can be a solution, a slurry, or a gel in a liquid.
Typically, the prepolymerization will be conducted using an olefin. A wide range of olefins can be used for the prepolymerization, preferably one selected from nonaromatic alpha-olefins, such as ethylene and propylene. It is within the scope of the invention to use a mixture of olefins, for example, ethylene and a higher alpha olefin can be used for prepolymerization. The use of a higher alpha olefin such as 1-butene with ethylene is believed to increase the amount of copolymerization occurring between the olefin monomer and the olefinically unsaturated portion of the metallocene.
The prepolymerization can be conducted under relatively mild conditions. Typically, this would involve using low pressures of the olefin and relatively low temperatures designed to prevent site decomposition resulting from high concentrations of localized heat. The prepolymerization typically occurs at temperatures in the range of about xe2x88x9230xc2x0 C. to about +110xc2x0 C., more preferably in the range of about +10xc2x0 C. to about +30xc2x0 C. The amount of prepolymer can be varied but typically would be in the range of from about 1 to about 95 wt. % of the resulting prepolymerized solid catalyst system, more preferably about 5 to 80 wt. %. It is generally desirable to carry out the prepolymerization to at least a point where substantially all of the metallocene is in the solid rather than in the liquid to maximize the use of the metallocene.
After prepolymerization, the resulting solid prepolymerized catalyst is separated from the liquid of the reaction mixture. Various techniques known in the art can be used for carrying out this step. After separating the solid from the liquid, the resulting solid is preferably washed with a hydrocarbon and then dried using high vacuum to remove substantially all the liquids and other volatile components that might still be associated with the solid. The vacuum drying is preferably carried out under relatively mild conditions, i.e. temperatures below 100xc2x0 C. More typically the prepolymerized solid is dried by subjection to a high vacuum at a temperature of about 30xc2x0 C. until a substantially constant weight is achieved. A preferred technique employs at least one initial wash with an aromatic hydrocarbon such as toluene, followed by several washes with a paraffinic hydrocarbon such as pentane, and then vacuum drying.
It is within the scope of the present invention to contact the prepolymerization reaction mixture product with a liquid in which the prepolymer is sparingly soluble, i.e. a counter solvent for the prepolymer, to help cause soluble prepolymer to precipitate from the solution. Such a liquid is also useful for the subsequent washing of the prepolymerized solid. The liquid mixture resulting from the prepolymerization of the inventive solid prepolymerized catalyst can be subjected to sonification to help break up particles if desired.
Further, if desired, the recovered solid prepolymerized catalyst system can be screened to give particles having sizes that meet the particular needs for a particular type of polymerization.
Another option is to combine the recovered inventive solid prepolymerized catalyst system with an inert hydrocarbon, such as one of the types used as a wash liquid, and then to remove that liquid using a vacuum. In such a process it is sometimes desirable to subject the resulting mixture to sonification before stripping off the liquid.
The resulting solid prepolymerized metallocene containing catalyst system is useful for the polymerization of olefins. Generally, it is not necessary to add any additional aluminoxane to this catalyst system. In some cases it may be found desirable to employ small amounts of an organoaluminum compound as a scavenger for poisons. The term organoaluminum compounds include compounds such as trimethylaluminum, trimethylaluminum, diethylaluminium chloride, ethylaluminum dichloride, ethylaluminum sesquichloride, and the like. Trialkyl aluminum compounds are currently preferred. Also in some applications it may be desirable to employ small amounts of antistatic agents which assist in preventing the agglomeration of polymer particles during polymerization. Still further, when the inventive catalyst system is added to a reactor as a slurry in a liquid, it is sometimes desirable to add a particulate dried solid as a flow aid for the slurry. Preferably the solid has been dried using one of the methods described earlier. Inorganic oxides such as silica or polyolefin xe2x80x9cfluffxe2x80x9d are particularly preferred. Currently, it is preferred to use a fumed silica such as that sold under the trade name Cab-o-sil. Generally the fumed silica is dried using heat and trimethylaluminum.
The solid catalyst system is particularly useful for the polymerization of alpha-olefins having 2 to 10 carbon atoms. Examples of such olefins include ethylene, propylene, butene-1, pentene-1, 3-methylbutene-1, hexenepropylene, 4-methylbutene-1, 3-methylpentene-1, heptene-1, octene-1, decene-1, 4,4-dimethyl-1-pentene, 4,4-diethyl-1-1hexene, 3,4-dimethyl-1-hexene, and the like and mixtures thereof. The catalysts are also useful for preparing copolymers of ethylene and propylene and copolymers of ethylene or propylene and one or more higher molecular weight olefins.
The polymerizations can be carried out under a wide range of conditions depending upon the particular metallocene employed and the particular results desired. Although the inventive catalyst system is a solid, it is considered that it is useful for polymerization conducted under solution, slurry, or gas phase reaction conditions.
When the polymerizations are carried out in the presence of liquid diluents obviously it is important to use diluents which do not have an adverse effect upon the catalyst system. Typical liquid diluents include propylene, propane, butane, isobutane, pentane, hexane, heptene, octane, cyclohexane, methylcyclohexane, toluene, xylene, and the like. Typically the polymerization temperature can vary over a wide range, temperatures typically would be in a range of about xe2x88x9260xc2x0 C. to about 300xc2x0 C., more preferably in the range of about 20xc2x0 C. to about 160xc2x0 C. Typically the pressure of the polymerization would be in the range of from about 1 to about 500 atmospheres or even greater. Thus inventive catalyst system is particularly useful for polymerizations carried out in particle form, i.e., slurry-type polymerization conditions.
The polymers produced in accord with this invention have a wide range of uses that will be apparent to those skilled in the art from the physical properties of the respective polymers. Applications such as moldings, films, adhesives, and the like are included. The present invention provides a method and catalyst system which can provide polyolefins having a broad molecular weight distribution.